multiplier phototube. The correlators derive servo intelligence not only 
as to which direction the beam shall be moved to intercept a contour but 
also as to which direction the beam shall move to follow a contour, once 
intercepted. 
The instrumentation of the stereoplotter, for performing search, 
trace, and printout of contours by servoing an electron beam, offers tremen 
dous possibilities for high-speed contouring and orthophoto mapping. Some 
of these possibilities have already been realized in this instrumentation, 
where contours are being traced at from 5 to 15 inches per second, at elec 
tronic system "C" factors of over 500. The present configuration and cir 
cuitry lend themselves to producing orthophotos, as well as contours, 
using the existing correlation circuitry and scanning system components. 
Since the system is basically an analog one,with several servo loops, and 
does not require a computer, it can eventually be produced at a much lcwer 
cost than could similar computer-controlled systems. 
The stereoplotter instrumentation configuration represents an at 
tempt to devise a system not limited in its plotting speed by the mechanical 
inertia of a tracing table. Therefore, image-processing techniques and 
electron-beam servoing techniques have been developed which enable the 
scanner electron beam to search for and lock on a contour, and follow it. 
If the scanner tube face covers the area of the neat model, no "X" or "Y" 
motion of the scanner tube is required, and a high-speed system results. 
Presently, the tube face covers only part of the neat model area, 
and the tube must be indexed to adjacent positions. 
SYSTEM DESCRIPTION 
The instrument may be considered to consist of two major systems: 
(1) a photoscan system capable of high-speed electronic scanning of a 
stereo diapositive pair and (2) a correlation system suitable for processing 
wide-band electrical signals. Additional subsystems are provided to inte 
grate the major systems and to protect the components against failure or 
overload. The basic instrumentation block diagram is shown in Figure 2. 
The photoscan system incorporates a modified Gamble/ Balplex 
plotter and a high-resolution electronic flying spot scanner capable of 
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